Apparently the best time to come up with new fiction ideas is when you are starting to work on something else. In purely procrastinatory rebellion, the brain latches on to every tiny detail in the world. “Ooh, look at this shiny thing! OOH WHAT IF THIS INSTEAD?”
It is kind of awesome, because it jogs loose all these ideas I can file away for later. Some will be crap. Some will be intriguing bits and pieces that never find a home in an actual plot. But others will flower and become the Next Big Project.
The trick is to write it all down and then get back to the original project. If you chase every shiny new thing when it first sounds exciting, you’ll write a whole lot of beginnings and no endings. Evidence: my trunk files are legion. I’m now a fan of always-finish-what-you-start because how else are you going to practice writing a whole arc? The ideas will wait, and probably turn out better for the percolating.